(1) Field
The disclosed methods and systems relate generally to control techniques, and more particularly to control systems for materials manufacturing processes such as semiconductor manufacturing processes.
(2) Description of Relevant Art
Lithography is a process used in semiconductor manufacturing to transfer a circuit pattern from a photomask or reticle to a semiconductor wafer, or more specifically, to transfer the photomask pattern to a layer of resist that has been deposited on the wafer surface, where the resist is sensitive to irradiation. Different types of lithography can be based on the wavelength of the radiation used to expose the resist. For example, photolithography, otherwise known as optical lithography, uses ultraviolet (UV) radiation and a corresponding UV-sensitive resist. Ion beam lithography uses a resist sensitive to an ion beam, electron beam lithography uses a resist film sensitive to a scanning beam of electrons to deposit energy therein, and X-ray lithography uses a resist sensitive to X-rays.
Photolithography employs a photomask that can be understood to be a quartz plate that is transparent to UV radiation and includes a master copy of an integrated circuit that is often a microscopic integrated circuit. The photomask can be used to block resist exposure to select areas using chrome opaque areas.
A stepper is a resist exposure tool used in many photolithography systems to expose part of the wafer or resist in a given exposure. Systems employing a stepper can require a xe2x80x9cstep-and-repeatxe2x80x9d process to expose the entire wafer as desired. A scanner is another type of resist exposure tool used in photolithography systems to expose part of the wafer or resist in a given exposure. Systems employing a scanner can require a xe2x80x9cstep-and-scanxe2x80x9d process to expose the entire wafer as desired. In the aforementioned systems, overlay can be understood as the superposition of the pattern on the mask to a reference pattern previously created on the wafer surface. Related to overlay is alignment, which can be understood to be including positioning, or aligning, the mask or reticle relative to markers or targets on the wafer, prior to the exposure. Accordingly, to achieve proper exposure, overlay and alignment, among other parameters, should be properly controlled.
The smallest transverse dimension of a developed photoresist can be known as the critical dimension (CD), depends on the exposure or photoresist exposure dose, which is a measure of the light absorbed by the photoresist. Accordingly, a proper exposure dose for a given pattern can include different exposure times for different substrates based on the substrate optical properties. For example, an exposure dose can be based on the photoresist layer thickness which can change during manufacture to alter the surface""s optical properties, thereby influencing the amount of light coupled into the photoresist. The CD of the developed photoresist thus determines the CD of the patterned material, and changes in a substrate""s optical properties can result in unacceptable variations during the manufacturing process.
As the demand for smaller yet more complicated integrated circuits (ICs) increases, there is a similarly increased level of integration and hence reduction in CD. Because lithography can occur repeatedly throughout IC fabrication, the CDs of the lines in the different patterns which are transferred should be precisely controlled throughout the fabrication process.
Techniques to control photoresist using a fixed exposure time employ calibration techniques using test wafers which are coated with photoresist and exposed with the pattern of interest using various exposure times and stepper focus conditions. These xe2x80x9csend-aheadxe2x80x9d wafers are examined to determine the resultant CDs in the photoresist and the optimal exposure and focus conditions for subsequent use on production wafers. Such a technique is an xe2x80x9copen loopxe2x80x9d process that does not account for changes in processes that may precede a lithography step. Further, such a technique introduces time delays in the manufacturing process.
The disclosed methods and systems include a method for controlling critical dimension (CD) by controlling exposure dose error in a process system. The method comprises measuring a measured exposure dose error (xe2x80x9cdose errorxe2x80x9d) based on an output(s) of the process system, normalizing the measured dose error based on a target exposure dose (xe2x80x9ctarget dosexe2x80x9d), and, providing an exposure dose to the process system, where the exposure dose can be based on a normalized dose error(s) which may or may not include aforementioned measured dose error. The target dose can thus be associated with a process system characteristic(s) and/or at least one measurement system characteristic(s) that can include, for example, a photomask, an exposure tool, and/or a process level. The measured dose error can thus be provided to a database, table, etc., and otherwise stored and/or associated with a process system and/or measurement system characteristic such as a photomask, an exposure tool, and a process level. This database (xe2x80x9cnormalization tablexe2x80x9d) can be queried using hierarchical wildcarding querying methods to provide a target dose for normalizing measured dose errors, to provide a target dose for normalizing actual doses from the process system, and to provide a target dose for converting a dose error to an exposure dose for input (e.g., control) to the process system.
The exposure dose provided to the process system can thus be based on a combination of normalized exposure dose errors, where such combination can include a weighted moving average of normalized exposure dose errors. The exposure dose can thus be based on such combination, which can be converted from a unitless (normalized) quantity to an exposure dose using a target exposure dose retrieved from the normalization table. The target exposure dose used to convert the normalized dose error can be the target exposure dose used to normalize the measured exposure dose error, or can be a different target exposure dose.
Accordingly, the methods include providing a database(s) and/or memory(s) and/or other means (e.g., xe2x80x9cnormalization tablexe2x80x9d) to associate a normalized exposure dose error with a target exposure dose, and to update a target exposure dose using such normalized exposure dose error.
The methods can include computing an exposure dose error by measuring a critical dimension based on an output(s) of the process system. Accordingly, a CD error can be computed by comparing a measured CD with a target CD, where in one embodiment, the comparison can be a difference measure. Such CD error can be converted to an exposure dose error using a dose sensitivity, where such conversion can include a scaling.
The method also includes generating an ideal exposure dose error based on the normalized exposure dose error and an (actual) exposure dose previously provided to the process system, where the exposure dose provided to the process system can be based on such ideal exposure dose error(s). The exposure dose provided to the process system can be, for example, based on a combination of ideal exposure dose error(s), such as a weighted moving average of at least two ideal exposure dose errors.
Accordingly, the normalization table""s target exposure doses can be based on a target critical dimension, a manually entered exposure dose, a focus and exposure matrix (FEM) associated with the process system, and/or at least one normalized measured exposure dose error. Accordingly, the database and/or table can have access to learning modules, filters, averaging techniques, etc., to determine, compute, update, or otherwise provide updated target exposure doses and associate such updated target doses with a process system and/or measurement characteristic. The updated target exposure doses can be based one or more normalized exposure dose errors, which may be associated with the same process system and/or measurement system characteristic as the target exposure dose for which the normalized exposure dose error(s) is updating. The methods can thus include providing one or more default and/or initial target exposure doses (referred to herein collectively as xe2x80x9cdefault target dosesxe2x80x9d) and associating such default target doses with one or more process system and/or measurement system characteristics, where such default target exposure doses can be updated based on the aforementioned filters, averaging techniques, and/or learning modules, using for example, normalized exposure dose errors, to provide an updated target exposure dose. The updated target exposure dose can thus be provided in response to a query of the normalization table.
Also disclosed are systems for controlling critical dimension (CD) in a process system, the system comprising a measurement system to provide a measurement(s) associated with a process system error(s), and, a control system to provide an exposure dose to the process system, where the exposure dose can be based on a normalized exposure dose error(s), where the normalized exposure dose error can be based on a target exposure dose associated with a process system characteristic(s) and/or a measurement system characteristic(s). The system can also include a normalization table to associate the target exposure dose with a process system characteristic(s) and/or a measurement system characteristic(s). Such characteristics can include, for example, a photomask, an exposure tool, and/or a process level.
The systems can thus include one or more processors with instructions to cause the processor(s) to compute a critical dimension (CD) error based on an output(s) of the process system. The CD error can be based on a comparison of a measured CD and a target CD, where the measured CD can be based on the output(s) of the process system. The control system can also include instructions to cause a processor to normalize a computed critical dimension (CD) error based on a target exposure dose, where the target exposure dose can be based on a manual entry(s), a normalized exposure dose error(s), and/or data based on a focus and exposure matrix(s) (FEM). Accordingly, the disclosed systems include instructions to cause a processor to retrieve the target exposure dose based on a query of a normalization table, where the normalization table associates the target exposure dose with a process system characteristic(s) and/or a measurement system characteristic(s).
The measurement system can be an exposure tool and/or a critical dimension measurement tool. The process system can include a lithography, a microlithography, a chemical mechanical polish (CMP), a diffusion, thin film, a metal deposition, an ion implantation, and/or an etching system.
The disclosed systems can include instructions to cause a processor to update a target exposure dose using at least one normalized exposure dose error, where the updated target exposure dose can be stored in the normalization table. The target dose can thus be updated using instructions that implement a learning module(s), a filter(s), and an averaging module(s). The updated target dose can thus be used for future normalizations of CD error(s).